PURPOSE: To surely fasten a retaining curved member by mounting a retaining member for closely pressing the retaining curved member to a back face of a front plate, on a fastening member comprising an enlargement body having an enlarging portion which is enlarged when a threaded pin is screwed therein.

CONSTITUTION: An enlarging portion 8 of an enlargement body 11 is engaged in a blind hole 9 of a front plate 1, a threaded pin 13 is screwed in the same to enlarge a taper-shaped inside screw thread 12 formed on the enlarging portion 8, and a stopper flange 10 of the enlargement body 11 is kept into contact with an edge of the blind hole 9. A retaining cap 14 as a retaining member is screwed in a projecting end 14 of the threaded pin 13 for surrounding the enlargement body 11 by the retaining curved member 2. Accordingly the retaining curved member 2 can be surely fastened to a back face of the front plate 1 by the retaining cap 15.